The closely-related problems of electromagnetic imaging and focusing beyond Abbe's diffraction limit (set by ˜λ/n where λ is the vacuum wavelength and n is the refractive index) have received considerable attention in the past decade, motivated largely by optical studies using sub-wavelength apertures to probe the near field. Techniques such as sharp tip imaging and far-field time reversal mirrors have been proposed to improve the focusing resolution, and factors as large as ˜100 have been achieved in the THz range.
Generally speaking, sub-wavelength focusing techniques involve the evanescent components of the field, i.e., the near field. Because of this, standard interference techniques and geometrical optics methods do not apply. More recent developments have centered on negative refraction techniques to study and to control the electromagnetic near-field. Examples of such work include examinations of perfect lenses as discussed by J. B. Pendry “Negative Refraction makes a Perfect Lens,” Phys. Rev. Lett. 85, 3966 (2000) and by R. Merlin, “Analytical Solution of the Almost-Perfect-Lens Problem,” Appl. Phys. Lett. 84, 1290 (2004), the experimental verification of negative refraction at microwave frequencies discussed by R. Shelby, D. R. Smith and S. Schultz, “Experimental Verification of a Negative Index of Refraction” Science 292, 77 (2001), and imaging with negative-refractive index slabs discussed by A. Grbic and G. V. Eleftheriades, “Overcoming the diffraction limit with a planar left-handed transmission-line lens,” Phys. Rev. Lett. 92, 117403 (2004) and negative permittivity slabs discussed by N. Fang, H. Lee, C. Sun and X. Zhang, “Sub-Diffraction-Limited Optical Imaging With a Silver Superlens” Science 308, 534 (2005).